Standby power modules and uninterruptible power supplies are generally used in data processing installations for handling short-duration interruptions of normal power supplies. However, in some types of installations, special arrangements must be made to avoid sudden power failures which can occur if the auxiliary power supply is exhausted or somehow interrupted. Any software system which makes use of cache buffering is susceptible to loss of data or loss of file due to a sudden power failure. It is therefore important, where possible, to anticipate a power failure of the standby power module, and to provide an orderly shut-down of the data processing installation before such failure occurs.